It is well known that LLDPE (linear low density polyethylene) such as copolymers of ethylene and hexene generally produce films of superior properties over high pressure highly branched low density polyethylenes (LDPE). LLDPE films exhibit higher tensile strength, outstanding puncture resistance and enhanced impact strength. However, the extrudability of LLDPE resins is poor mainly because of their high shear viscosity. Due to the absence of long chain branching in LLDPE, its shear viscosity exhibits less shear thinning than branched LDPE. This non-Newtonian rheological behaviour brings about extrusion difficulties by increasing extrusion head pressure resulting in high motor amperage and high torque which limits the output under a given set of extrusion conditions compared to that possible with prior art LDPE resins.
Furthermore, the extrusion problem is frequently accompanied by the appearance of surface irregularities on the produced film. Surface irregularities or more generally, "melt fracture" occur over a range of shear rate depending on the molecular characteristics of the polymer. These are characterized by closely spaced circumferential ridges along the extrudate when extruded through a capillary die. In a more severe form it resembles what is generally known as "sharkskin." The onset of melt fracture is believed to occur at or above a critical shear stress although the concept is yet to be fully understood.
In the past many attempts were made to alleviate the extrusion problems encountered with LLDPE. For example, blends of linear low density polyethylenes with LDPE (branched high pressure low density polyethylene) are known. The use of various kinds of processing aids such as low molecular weight hydrocarbons, fatty amides or bis-amides, fluoroelastomers and certain silicon polymers is also known.
In a copending application Ser. No. 810,117 filed Dec. 18, 1985, now abandoned, in which I am a co-inventor and which is incorporated herein by reference various blends of LLDPE with high MI resins including LMW-HDPE are described. In accordance with this invention further improvements in blends of LLDPE and LMW-HDPE are obtained by adding HMW-HDPE.